1. Field of the Invention
The present invention relates to a piezoelectric oscillator, and in particular to a piezoelectric oscillator which is excellent in noise characteristic and aging characteristic.
2. Description of the Related Art
As a crystal oscillator used in reference signal sources of communication devices, one having a circuit configuration as shown in FIG. 3 is known.
In FIG. 3, a circuit 101 surrounded by a broken line is a typical Colpitts oscillation circuit. A crystal resonator 102 is connected to base of a transistor 103. The base of the transistor 103 is connected to its emitter via a capacitor 104. The emitter of the transistor 103 is connected to ground via a parallel circuit of a capacitor 105 and a resistor 106. Collector of the transistor 103 is connected to a power supply Vcc. Furthermore, one terminal of the crystal resonator 102 is connected to ground via a series circuit of a capacitor 107 and a trimmer capacitor 108.
Incidentally, oscillation output is taken out from ends of the trimmer capacitor 108.
Further, an amplification circuit 109 surrounded by a broken line in FIG. 3 forms a part of an AGC circuit in conjunction with a rectification circuit described later. Emitter of a transistor 110 is connected to ground. Collector of the transistor 110 is connected to the power supply Vcc via a resistor 111. The collector of the transistor 110 is connected to its base via a resistor 112. Furthermore, the base of the transistor 110 is connected to the emitter of the transistor 103 via a capacitor 113.
Furthermore, a circuit 114 surrounded by a broken line in FIG. 3 is the above described rectification circuit. Anode of a diode 115 is connected to cathode of a diode 116. Cathode of the diode 115 is connected to ground. Anode of the diode 116 is connected to the power supply Vcc via a resistor 117. The anode of the diode 116 is connected to ground via a capacitor 118. The cathode of the diode 116 is connected to the collector of the transistor 110 via a capacitor 119. The anode of the diode 116 is connected to the base of the transistor 103 via a resistor 120 in a feedback form.
Incidentally, a capacitor 122 shown in FIG. 3 is a bypass capacitor.
Operation of the crystal oscillator having such a configuration will now be described.
Since the oscillation circuit 101 is a typical Colpitts oscillation circuit as described above, description thereof will be omitted.
An output signal produced from the emitter of the transistor 103 on the basis of parameters of the oscillation circuit 101 is supplied to the amplification circuit 109, amplified therein to a required level, and then generated from the collector of the transistor 110.
The output signal amplified in the amplification circuit 109 is supplied to the diode 115 and the diode 116 of the next stage via the capacitor 119.
A signal component of half a cycle of plus side of the output signal supplied to the rectification circuit 114 flows through the diode 115, and the capacitor 119 is charged with electric charge so that the anode side of the diode 115 will have a lower potential. A signal component of half a cycle of minus side of the output signal supplied to the rectification circuit 114 flows through the diode 116. At the anode of the diode 116, therefore, there is generated a potential equivalent to the sum of the potential generated by the signal component in the half cycle of minus side and the potential generated by the charged electric charge of the capacitor 119. The resultant potential is applied to the base of the transistor 103 included in the oscillation circuit 101.
In this way, the base potential is changed by the control of the AGC circuit. As a result, the collector output of the transistor 103 changes.
When the level of the output signal of the oscillation circuit 101 becomes high, the difference (p--p value) between a minimum value and a maximum value of a signal supplied from the amplification circuit 109 to the rectification circuit 114 increases, and the absolute value of the potential of negative polarity generated on output of the rectification circuit 114 increases. Therefore, the base bias voltage of the transistor 103 falls, and the level of the output signal of the oscillation circuit 101 falls.
On the other hand, in the case where the level of the output signal of the oscillation circuit 101 has fallen, operation opposite to the above-described operation is caused. Therefore, description of its operation will be omitted.
By conducting the above-described operation repetitively, therefore, the crystal oscillator outputs a stable level signal on the basis of the set conditions thereof.
In such a highly stable crystal oscillator as to use an AGC circuit, the aging characteristic especially poses a problem. In order to obtain a favorable aging characteristic, circuit constants are set so as to keep the level of the excitation current of the crystal resonator at a low level.
That is, the excitation current of the crystal resonator is kept at a low level by making the base bias current of the transistor 103 small and holding down the gain of the transistor 103 to a low value.
If the transistor 103 is driven with a low base bias current, however, then the collector current necessarily becomes low. As evident from a relationship between the collector current and the noise figure shown in FIG. 4, therefore, there is a problem that the noise figure becomes high.
In other words, it is necessary to set the collector current to a value of a point B for making the noise figure low. For increasing the collector current, however, the base bias current must be increased. As a result, the gain of the transistor 103 becomes larger than need be. The excitation current level of the crystal resonator thus becomes high. Accordingly, a favorable aging characteristic cannot be obtained.
In the conventional crystal oscillator, therefore, the aging characteristic is given priority and the collector current is set to a value of a point A having an extremely low collector current. Thus, the rise of the noise figure is ignored.
An object of the present invention is to provide a crystal oscillator capable of having a favorable aging characteristic and improved in noise characteristic.